British light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the styles failed to mesh, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor stated in the ringside as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Secures Victory in Slow Affair
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger slipping to the canvas several times during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry contest to capture the European light-heavyweight title.
The venue grew hushed for a extremely emotional tribute ahead of the contest as ten bells rang in remembrance of the city's native Ricky Hatton, who passed away 46 years old in September.
It was the first major televised match card in the city since Hatton's passing. As the last bell faded, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The fighting within the ring equaled the emotion. The eventual winner was floored by a left hook directly to the chin in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling developing under his face as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur narrowly won the result with judges scorecards of even score, narrow lead and wider gap, but each competitor were relentless and courageous, everything their hero the legend represented.
